Raikkonen’s best lap of 1m 23.477s put him almost a second up on nearest rival Daniil Kvyat in the Red Bull. They were the only two drivers to set their times on Pirelli's new ultrasoft tyre compound.
Despite his table-topping effort, Raikkonen admitted he was still unsure as to the SF16-H's ultimate potential.
"I don't know if this is going to be a good car," he said. "No one can tell, but it seems to be a good car even if there are still many things to try out. We don't look at what the other teams do, we do our stuff and get ready. It's up to us to make sure we are in a good position. There is a good potential but we can still improve."
Force India tester Alfonso Celis was an impressive third fastest, his best supersoft-tyred run yielding a time 1.3s down on Raikkonen's best. The rest of the field clocked their quickest laps on either the soft or medium compound.
After troubles earlier in the week, Renault enjoyed a day of excellent reliability with Kevin Magnussen racking up the day's highest lap total - 153 - en route to fourth in the time sheets, just ahead of Toro Rosso's Max Verstappen and Felipe Nasr in the 2015 Sauber.
“I certainly have had a workout over the last couple of days with over 260 laps completed!" quipped Magnussen. "It feels so good to be back in an F1 car and working through the programme and making strong progress. Our race simulation went well and I’m really gelling with the team; it’s a real pleasure to be driving here and I can’t wait to get back in the car on Tuesday.”
Manor rookie Rio Haryanto had an encouraging morning for Manor, only to lose control and nudge his MRT05 into the barriers at Turn 4 early in the afternoon. The Indonesian driver thus finished 11th, but was still happy with his start to the 2016 pre-season.
“It’s been an important week for me and the team here in Barcelona and generally it’s gone well," he said. "I’ve learned a lot about the new car and identified some areas for improvement, to make the second test even more positive for me.
"I did make a mistake this afternoon and the damage to the car could not be repaired quickly, so it’s a shame we didn’t complete our programme today. Generally though, we’ve achieved a lot of mileage and completed some good evaluation work."
The only man behind Haryanto was Fernando Alonso, whose day was limited to a frustrating three laps due to technical problems on his McLaren.
Thursday's unofficial testing times from Barcelona
|1||Kimi Raikkonen||Ferrari SF16-H||1:23.477||78|
|2||Daniil Kvyat||Red Bull Racing-TAG Heuer RB12||1:24.293||0.816||94|
|3||Alfonso Celis||Force India-Mercedes VJM09||1:24.840||1.363||75|
|4||Kevin Magnussen||Renault R.S.16||1:25.263||1.786||153|
|5||Max Verstappen||Toro Rosso-Ferrari STR11||1:25.393||1.916||110|
|6||Felipe Nasr||Sauber-Ferrari C34||1:26.053||2.576||121|
|7||Nico Rosberg||Mercedes F1 W07 Hybrid||1:26.187||2.710||86|
|8||Lewis Hamilton||Mercedes F1 W07 Hybrid||1:26.295||2.818||99|
|9||Felipe Massa||Williams-Mercedes FW38||1:26.483||3.006||54|
|10||Esteban Gutierrez||Haas-Ferrari VF-16||1:27.802||4.325||89|
|11||Rio Haryanto||Manor Racing-Mercedes MRT05||1:28.266||4.789||51|
|12||Fernando Alonso||McLaren-Honda MP4-31||No time||3|